In the field of friction materials, it is generally known to use substrates made from porous materials to manufacture friction members, such as friction brake disks. The manufacture of such friction members generally begins with the construction of a porous preform, such as an annular preform.
The annular preforms can be constructed using several different known methods. For example, carbon fiber fabric plies can be needled together and annular preforms can be cut from the stacked material. The plies may be made, for example, from airlaid fibers or woven fibers. Also, near net shape preforms can be formed, for example, by braiding the carbon fiber into a desired shape. Certain carbon fiber fabrics are known having a weave that facilitates laying the fabric in a spiral form. In this context, “near-net” refers to forming structures having a form close to a desired shape of the final article, such as an annular brake disk.
Oxidized polyacrylonitride (“PAN”) fibers or pitch-based fibers are common examples of starting fibers used in this type of application. Subsequently, these fibers may be carbonized in a high temperature treatment step. In another conventional approach, the starting fibers are formed using a resin or pitch, and the resultant mass is later cured with a reactive gas, such as nitrogen gas. The thusly cured mass is then carbonized to obtain a semi-rigid preform.
In any event, it is desirable to further density the resulting porous preform (especially with a carbonaceous material) so as to obtain desired friction and mechanical properties.
Chemical vapor infiltration (“CVI”) is a conventional densification technique for obtaining carbon/carbon (sometimes referred to in the art as “C/C”) composite materials. CVI typically uses a hydrocarbon-containing gas to infiltrate a porous preform. The CVI gas is then cracked under high temperatures so as to leave a carbon coating on the fiber structure of the preform, thereby increasing the density of the article.
CVI using a gaseous precursor typically requires several hundred hours of processing in order to obtain a carbon/carbon structure having desired density characteristics and mechanical properties. By way of example, a typical CVI process includes a first gas infiltration cycle performed, for example, over approximately 300-500 hours or more.
However, conventional CVI frequently causes rapid blockage of the surface porosity of the preform before interior portions of the preform are adequately densified. In order to “reopen” the surface porosity (to allow the gaseous precursor to continue to reach interior parts of the article), an intermediate machining step becomes necessary. In general, this intermediate machining (using a known method, such as milling) removes surface layers of the preform having carbon-blocked pores to expose open pores of the preform, so that the hydrocarbon gas can again infiltrate the preform structure. Taking into account that several hundred preforms are densified in a typical densification, the intermediate machining step can add as much as 48 hours to the overall CVI densification process.
Once the intermediate machining of the partially densified articles is completed, a second CVI process is performed to make use of the reopened surface porosity of the preforms, which can last, for example, another 300-500 hours or more. This generally completes the densification process.
Another approach to densifying porous preforms uses a liquid instead of gaseous hydrocarbon precursor. This method of densification is sometimes referred to in the art as “film boiling” or “rapid densification.”
The use of liquid precursors for densification is discussed in, for example, U.S. Pat. Nos. 4,472,454, 5,389,152, 5,397,595, 5,733,611, 5,547,717, 5,981,002, and 6,726,962. Each and every one of these documents is incorporated herein by reference in its entirety in all venues and jurisdictions where incorporation by reference is permitted.
Film boiling densification generally involves immersing a porous preform in a liquid, particularly a liquid hydrocarbon, so that the liquid substantially completely infiltrates the pores and interstices of the preform. Thereafter, the immersed preform is inductively heated to a temperature above the decomposition temperature of liquid hydrocarbon (typically 1000° C. or more). More particularly, the liquid hydrocarbon adjacent to the inductively heated preform structure dissociates into various gas phase species within the preform porosity. Further thermal decomposition of the gas phase species results in the formation of pyrolitic carbon on interior surfaces in the open regions of the porous material.
The liquid hydrocarbon may be cyclopentane, cyclohexane, 1-hexene, gasoline, toluene, methylcyclohexane, n-hexane, kerosene, hydrodesulfurized kerosene, benzene, or combinations thereof. In addition, the liquid precursor may contain an organosilane, such as methyltrichlorosilane, dimethyldichlorosilane, methydichlorosilane, or tris-n-methyl amino silane. In some cases, the liquid precursor may be a mixture of an organosilane and a hydrocarbon.
The liquid precursor may be formulated in a known way to obtain combination decomposition products. For example, the decomposition product may comprise silicon carbide and silicon nitride, or carbon/silicon carbide or carbon/silicon nitride.
Because of the boiling liquid surrounding the preform, a strong thermal gradient develops between the inner (i.e., core) and the outer (i.e., peripheral) parts of the disks. Densification generally starts at core regions because the temperature there is relatively higher than at more outward surface parts. The porous article can therefore be substantially completely densified in only one densification process step, much faster than when using the regular isobar CVI (“I-CVI”) process (where densification preferably occurs at surfaces of the articles), which tends to seal the porosity of the article and prevent further gas infiltration. The kinetic of liquid precursor densification may be on the order of 100 times faster than using a gas infiltration step.
However, because the preform is immersed in a liquid, a high power level is necessary to keep the maximum temperature above the cracking temperature of the liquid precursor. For example, in the case of densification using cyclohexane as a precursor, an interior temperature of the porous article during densification may be between about 900° C. and about 1200° C. although the surrounding liquid cyclohexane temperature is only about 80° C. to about 82° C. As a result, overall electric consumption is high compared to the standard l-CVI process.
Also, as the densification front moves toward the peripheral edges/surfaces of the porous preform, the power must be progressively increased in order to maintain a necessary temperature of the densification front. Thus, at the end of a densification cycle, the power level might be 5 or 10 times that of the initial power level. This increases the electric consumption and necessitates costly power supplies able to deliver the required heating.
Certain conventional solutions to these problems have been proposed in U.S. Pat. Nos. 6,994,886 and 5,981,002. For example, the preforms can be produced so as to be oversized, and densification is stopped when the densification front is still slightly (e.g., a few millimeters) away from the surface of porous article. This approach decreases the power needed to heat the core of the preform because the preform itself effectively acts as an insulator—the thicker the preform is, the better it serves to insulate its interior relative to the above-described thermal gradient. Also, the required final power to achieve the densification will be lower, depending on the thickness of sacrificial material (i.e., the depth of the undensified material at the surface of the preform). However, this approach necessary results in a certain level of material waste. In addition, when thicker preforms are used, infiltration as a whole becomes relatively more difficult. This can cause the core part of the preform to be insufficiently densified because the precursor has difficulty in reaching the interior of the preform.
Another approach relates to wrapping a preform with another material to create a physical boundary between the boiling liquid precursor and the preform itself. Depending on the nature of the layer, different results are expected. In U.S. Pat. No. 5,981,002, a layer of carbon felt is proposed to improve the edge densification of the disk. The carbon felt allows the densification front to move closer to the edge (i.e., surfaces) of the preform using less power. The carbon felt can withstand the high temperature of the densification front as it approaches the surface of the preform.
In certain situations, this approach cannot be used. For example, when the preform is inductively heated by electromagnetic coupling, the carbon felt itself may be inductively heated (like the preform) and become densified during the densification cycle. This would seal the porosity of the article, preventing the precursor from reaching interior parts of the preform and compromising the proper densification of the disk.
U.S. Pat. No. 6,994,886 discloses using one or more layers of a polytetrafluoroethylene (PTFE) textile (sometimes commercially known under the trademark Gore-Tex®). This document asserts that liquid entry into the preform is limited by PTFE so the required electrical power to density the material is significantly decreased and the densification rate increased. However, because of the low permeability of the PTFE textile (compared to the permeability of carbon felt, for example) the transfer of precursor to interior parts of the preform is hindered. Accordingly, when articles being densified are relatively thick, there is depletion of precursor at the core portions of the preform. This can result in an insufficiently densified core (sometimes referred to as a “hollow” core).
For example, if a 25 mm thick carbon brake disk preform is to be densified, the use of PTFE textile to insulate the preform necessitates a dramatic decrease in the densification kinetic (i.e., slower densification) in order to avoid a hollow core. Thus, the benefits of using PTFE as disclosed in U.S. Pat. No. 6,994,886 have to be balanced by an increase of the cycle time.
In view of the foregoing, an improved approach to the use of liquid precursors for densifying porous articles is desirable.